Acoustic wave sensors are utilized in a number of sensing applications, such as, for example, temperature, pressure, humidity and/or gas sensing devices and systems. Examples of acoustic wave sensors include devices such as acoustic wave sensors, which can be utilized to detect the presence of substances, such as chemicals and biological materials. An acoustic wave (e.g., SAW/BAW) device acting as a sensor can provide a highly sensitive detection mechanism due to the high sensitivity to surface loading and the low noise, which results from their intrinsic high Q factor.
Surface acoustic wave devices are typically fabricated using photolithographic techniques with comb-like interdigital transducers placed on a piezoelectric material. Surface acoustic wave devices may have either a delay line or a resonator configuration. The selectivity of a surface acoustic wave chemical and/or biological sensor is generally determined by a selective coating placed on the piezoelectric material. The absorption and/or adsorption of the species to be measured into the selective coating can cause mass loading, elastic, and/or viscoelastic effects on the SAW/BAW device. The change of the acoustic property due to the absorption and/or adsorption of the species can be interpreted as a delay time shift for the delay line surface acoustic wave device or a frequency shift for the resonator (BAW/SAW) acoustic wave device.
Acoustic wave sensing devices often rely on the use of quartz crystal resonator components, such as the type adapted for use with electronic oscillators. In a typical gas-sensing application, the absorption of gas molecules in a selective thin film coating (i.e., applied to one surface of the crystal) can increase the mass of the crystal, while lowering the crystal's resonant frequency. The frequency of a thickness shear mode (TSM) crystal unit, for example, such as an AT-cut unit, is inversely proportional to the thickness of the crystal plate. For example, a typical 5-MHz 3rd overtone plate is on the order of 1 million atomic layers thick. The absorption of analyte is equivalent to the mass of one atomic layer of quartz, which changes the frequency by approximately 1 ppm.
The thickness-shear-mode resonators are therefore widely referred to as a quartz crystal microbalance. Calculations have determined that the sensitivity of a fundamental mode is approximately 9 times more sensitive than that of a 3rd overtone. A 5 MHz AT-cut TSM crystal blank, for example, is approximately 0.33 mm thick (fundamental). The thickness of the electrodes can be, for example, in a range of approximately 0.2–0.5 μm. The change in frequency due to the coating is typically: ΔF=−2.3×106 F2 (ΔM/A), where the value ΔF represents the change in frequency due to the coating (Hz), F represents the frequency of the quartz plate (Hz), ΔM represents the mass of deposited coating (g), and the value A represents the area coated (cm2).
Selective adsorbent thin film coated acoustic sensors such as, for examples, quartz crystal resonators, surface acoustic wave and quartz crystal microbalance devices are attractive to humidity, gas and chemical detection applications because of their high sensitivity, selectivity and ruggedness. The detection mechanism implemented depends on changes in the physicochemical and electrical properties of the coated piezoelectric crystal when exposed to humidity, gas and the like. Measurement results are usually obtained as the output frequency of a loop oscillator circuit, which utilizes a coated crystal as the feedback element.
When the sensor is exposed to analytes, the thin film adsorbs the analytes, and a corresponding frequency shift is measured as a result of any physicochemical and electrical changes. Factors that contribute to the coating properties include coating density, coating modulus, substrate wetting, coating morphology, electrical conductivity, capacitance and permittivity. Coating materials selection, coating structures and coating techniques affect the sensors' responses.
Conventional techniques for thin film deposition vary extensively, depending on the properties of the coating materials and substrate. Examples of such techniques include CVD, PVD, and sol-gel for most of the inorganic and composite materials. For polymeric materials, self-assembly dipping methods, casting, spray coating, and/or spin coating from a solution of the polymer in a volatile solvent are often preferred. Configurations based on these conventional techniques generally determine the properties of an acoustic wave sensor. Coating methods are also important for a sensor's repeatability. Because of their relatively short lifetimes, such sensors are replaced more often than those based on metal oxide. When sensors are replaced, they lose their memory of previously learned odors. In other words, the response curves of such devices vary, and the replacement sensors must then be retrained and/or recalibrated.
For practical reasons, zeolites are widely utilized as the physi-sorption coating materials. Zeolites are crystalline alumino-silicates of alkali or alkaline earth elements (e.g., Li, Na, K, Mg, Ca, Ba) with frameworks based on extensive 3-dimentional networks of AlO4 and SO4 tetrahedra. These tetrahedra are assembled into secondary polyhedral building blocks such as cubes, octahedral and hexagonal prisms. The final zeolite structure consists of assemblages of the secondary blocks into a regular, 3-dimentional crystalline framework. Each aluminum atom, for example, possesses a (−1) charge and this gives rise to an anionic charge in the network.
Cations are necessary to balance the charge and occupy non-framework positions. Typically the framework is composed of a regular structure of interconnected cages and/or channels. These devices or systems of essentially “empty” cages and/or channels provide the high storage capacities necessary for good adsorbents. Zeolite adsorbents are characterized by their uniform intra-crystalline aperture sizes. The uniformly sized apertures enable molecular discrimination on the basis of size (e.g., steric separation). Molecules larger than the maximum size that can diffuse into the crystal are excluded.
The sorption capacity and selectivity can be significantly affected by the type of cation used and the extent of ion exchange. This type of modification is important in optimizing zeolites for gas separation. The uniform pore structure, ease of aperture size modification, excellent thermal and hydrothermal stability, high sorption capacity at low partial pressures, and modest cost have made zeolites widely used in many separation application. For example, a selective adsorbent thin film coated quartz crystal microbalance chemical sensor can be utilized for the selective detection of CO. The thin coating comprises a solid non-porous inorganic matrix and porous zeolite crystals contained within the inorganic matrix, the pores of the zeolite crystals selectively adsorb chemical entities of a size less than a preselected magnitude.
The matrix can be selected from the group of sol-gel derived glasses, polymers and clay. The pores of the zeolite crystals are modified so as to be Lewis or Bronsted acidic or basic and capable of providing intrazeolite ligation by the presence of metal ions. The film can be configured from an alumina, boro-alumino-silicate, titanium, hydrolyzed diethoxydiphenyl silane, or silane rubber matrix containing zeolite crystals. The thickness of the inorganic matrix is generally about 0.001–10 μm and the diameter of the pores of the zeolite crystals is approximately 0.25–1.2 nm. The coating is a single layer of zeolite crystals protruding from an amorphous SiO2 matrix.
A polymer can be defined as a compound consisting of a large number of repeating units, called monomers. These monomers are joined together by covalent bonds to form a long chain. The degree of polymerization is defined as the number of repeating units in the chain. The properties of the polymer depend on the overall size of the polymer chain and on the inter- and intra-molecular forces that hold the polymer together. In general, the polymer properties of interest can be characterized as diffusion/permeation properties or as mechanical properties. The measurement of diffusion/permeation properties is straightforward when diffusion of a species into a polymer film produces a simple mass-loading effect. Polymers used as sensor coatings are butyl rubber, cellulose polymers, polysiloxanes, polyaniline and polyethylene, and the like.
Polymers, specifically rubbery, amorphous polymers, have several inherent advantages as chemically sensitive sensor coatings. They can be deposited as thin, adherent, continuous films of fairly uniform thickness by solvent casting or spray coating. They are nonvolatile and of homogeneous composition, and their chemical and physical properties can be modified to some extent by judicious choice of monomers and synthetic procedures. The glass transition temperature Tg is the temperature at which a polymer changes from glassy to rubbery. Above Tg, permeability is governed entirely by diffusion forces and adsorption proceeds rapidly and reversibly. One more advantage of rubbery, amorphous polymers is that their sorption isotherms are often linear over relatively large ranges in penetrant concentration.
One of the disadvantages with polymers, however, such as a polyimide film used in most humidity sensors, is that repeated heating can delaminate the polymer or polyimide film due to thermal expansion coefficient mismatch with the substrate and absorption in general. Heating therefore can cause irreversible change to the polyimide film. In high condensing applications (e.g., a fuel cell), a sensor that relies upon a polymer or polyimide film cannot meet these requirements.